So the public doesn't know yet that Superman is from Krypton? I figured it was known already...

No they don't. As explained in Guardian (aka the first issue of the series/season), six months had passed since Apokolips came to Earth (the event is known as "Contact"), and it is said that the fact that aliens exist has caused a mixed sense of concern, wonder, and fear. But in Metropolis, the city and its citizens feel safe because of, you guessed it, Superman.
Later, Clark states to Lois that he'd like to finally tell the world that Superman is an alien, but she goes against the idea because there is concern brought on by the pictures of the aliens on Apokolips, and therefore it might not be the best time, especially with Lex Luthor making such noise about it in public. And of course, in that issue, Lex was talking to General Lane over Earth's defense against another alien threat, thus the need for his Guardian orbital weapons platform. When Lane mentions the fear that weapons in space could easily be turned on the planet, Lex counters that with a question of how Superman can be viewed as any less dangerous and what is the world to do if he decides to turn his back on it or prove a threat.

So yeah, they don't. But it seems to have been setting up the reveal since the first episode, and because you have the in-development JLA in the background with Martian Manhunter, it would make sense that the revelation would have to be addressed before the JLA is fully established.